The possibility that an endogenous endo-dextranase contributes to the structural complexity of the adherent and insoluble glucan synthesized from sucrose by oral streptococci is currently under investigation. For this purpose, Streptococcus salivarius mutants that produce either more or less dextranase have been isolated. Whereas the high-dextranase mutant synthesized about the same amount of insoluble glucan and produced adherent deposits like the parent, the low-dextranase mutant produced more soluble glucan and did not form adherent deposits. Two dextranase-deficient mutants of S. mutants 6715-13 have been identified and these also synthesized more soluble glucan. In order to determine whether these phenomena are causally related, the amounts of the glucosyltransferase enzymes producing soluble and insoluble glucan will be assessed. The possible role of plasmids in the physiology of S. mutants is being investigated also. As a result of these studies, two new species of circular DNA have been isolated from strain LM-7. The larger one is a 30Mdal covalently closed molecule, and the smaller one is a 2Mdal circular species, which is not a supercoil and which appears to originate from the previously described 3.6Mdal pAM7 plasmid.